The compound of formula 7 is not only an important moiety forming N-terminal group of a pharmaceutical raw material (ref. Korean Patent Application Nos. 1998-0060266 and 1999-0033490; WO 00/39124) developed as a novel thrombin inhibitor for oral administration, but also one moiety of a compound (ref. International Application No. PCT/US95/02558; WO 95/23609) developed and effectively used for prohibition of blood coagulation and treatment of thrombosis of a mammal. Therefore, more attention is gradually drawn to preparing the compound of formula 7 above.